The development of the Liquid Metal Fast Breeder Reactor LMFBR requires a highly reliable method of controlling the liquid sodium cooling flow through the reactor core. One such reactor would use an annular flow of sodium adjacent the uranium core and a surrounding annular flow of sodium through a fluidized bed of tantalum spheres. The tantalum spheres serve to poison the nuclear reaction when they drop because of a substantially reduced flow of sodium. A satisfactory controller for such a reactor must reduce the flow through the fluidized bed to drop the tantalum balls and poison the reaction in the following two critical situations: (1) when the flow in the reactor decreases, e.g., due to pump failure and (2) when the temperature of the liquid sodium increases, e.g., due to excessive power generation.